The invention relates to a trip device for an electric switch, comprising a yoke of magnetizable material, a permanent magnet immovably positioned with respect to the yoke and a movably supported armature of magnetizable material, mutually arranged in a manner such that the armature, the permanent magnet and the yoke form a first magnetic circuit, the armature being able to assume a first position under the influence of the magnetic field of the permanent magnet, further comprising at least one magnet winding and spring means for causing the armature to assume a second position in response to the magnetic field generated by an electric current, flowing during operation in the at least one magnet winding, if a pre-set threshold value is exceeded, a second magnetic circuit being provided for adjusting the threshold value in the form of shunt means of magnetizable material interacting with the yoke and the permanent magnet in order to influence the magnetic field in the first magnetic circuit by shunting.
A trip device of this type which is suitable for activating the switch mechanism of switches by electrical means is known per se from U.S. Pat. No. 3,693,122.
In the nonactivated operating state, the armature is held in the first position against the force action of the spring means under the influence of the permanent magnet, the switch to be actuated by the trip device can be in the conducting state, for example. By energizing the at least one magnet winding with an electric current the magnetic field of the permanent magnet acting on the armature can be influenced in a manner such that the armature is moved to its second position under the influence of the spring means in order to bring the respective switch into the nonconducting state, for example. In practice, this may be the case if earth fault currents occur or if a current to be monitored in an electrical installation exceeds a predetermined maximum value.
To detect these fault situations, separate means, for example an electronic circuit designed for this purpose, may be employed and with the aid thereof the at least one magnet winding can be energized for causing the armature to assume the second position. A trip device equipped with such an electronic circuit is, for example, disclosed in U.S. Pat. No. 4,731,692.
If the use of a separate electronic circuit and/or energizing circuit for the at least one magnet winding is undesirable from, for example, a cost engineering point of view or because of a greater chance of malfunctions, a current to be monitored or a derived value thereof can, of course, also be passed directly through the at least one magnet winding of the trip device.
The threshold value above which the trip device responds is influenced by mechanical tolerances in the spring means and the dimensions of, for example, the yoke, the supporting means for the armature and the like, as a result of which undesirable air gaps and magnetic leakage fields may be produced in the first magnetic circuit, and by tolerances in the magnetic field strength of the permanent magnet.
For setting the threshold value, the magnetic shunt means can advantageously be used in order to influence the magnetic field strength of the permanent magnet acting on the armature by magnetic shunting.
However, in addition to a desired influencing of the magnetic field in the first magnetic circuit, said shunt means also cause undesirable leakage fields which may disadvantageously influence the intended accuracy of the set threshold value of the trip device itself. The setting of adjacently mounted trip devices, for example, in a three-phase installation, or other electrical devices sensitive to magnetic fields may also be influenced.
U.S. Pat. No. 3,693,122, for example, discloses an embodiment of a trip device in which the shunt means have essentially the form of a flat plate, which shunt plate is positioned with its one surface partially opposite a pole face of the permanent magnet and with its other surface partially opposite the armature, while another section of the shunt plate is arranged at a distance from, and at an angle to, a part of the yoke.
The rectangular form, essentially used, of the known shunt plate is undesirable as regards the occurrence of magnetic leakage fields to the environment of the trip device. In particular, this happens when relatively flat permanent magnets are used because a relatively large magnetic leakage field is then produced between the shunt plate and the part of the yoke on which the permanent magnet is mounted, due to the relatively large surface area of said shunt plate.
The use of shielding means against the undesirable leakage fields, for example, in the form of a metal housing or metal screens, will in practice often entail an increase in the total cost of the trip device or a switch provided with such a trip device. In practice, it is moreover not always possible to use such screening means, for example, as a consequence of the construction of the trip device, but also in situations in which no metallic housings are permitted for safety considerations.